The invention relates to a method for producing a steel melt containing up to 30% manganese.
In this context steel melts are considered which may contain, in addition to Mn, up to 1.5% C, up to 22% Al, up to 25% Cr, up to 30% Ni, as well as up to 5% of each of Ti, V, Nb, Cu, Sn, Zr, Mo and W, as well as up to 1% of each of N and P.
Such steels are used, for example, in the automotive industry in order to meet the requirement for reducing the fleet fuel consumption while maintaining the highest comfort level. Weight reduction plays a crucial role. The supplier tries to meet this desire, in particular in the automobile body segment, by attempting to reduce the wall thickness through the use of higher-strength steels, without suffering losses in the buckling resistance, in the shaping process by deep drawing and/or stretch drawing, and in the coating process.
EP 0 889 144 A1 proposes a cold-formable, in particularly readily deep-drawable austenitic lightweight steel with a tensile strength of to 1100 MPa. The major elements of this steel are Si, Al and Mn in a range of 1 to 6% Si, 1 to 8% Al, and 10 to 30% Mn, with balance iron, including usual steel-accompanying elements.
The attainable high deformability is attained through particular effects, for example TRIP (Transformation Included Plasticity), TWIP (Twinning Included Plasticity) or SID (Shearband Included Plasticity) properties of the steel.
The problem with these steels is their metallurgical production due to the high contents of alloying elements.
DE 101 64 610 C1 disclose a method for producing a steel melt containing up to 30% manganese, wherein the charge material is melted into a melt, for example, in an induction furnace, wherein the charge melt is deoxidized with aluminum such that the oxygen is bound during the entire melting process by the aluminum, and manganese and silicon are added to the deoxidized melt, and the temperature of the molten bath is maintained slightly above the liquidus temperature. This approach is intended to prevent the creation of brown smoke when manganese is added to the melt.
DE 35 02 542.5 discloses a production method for a steel with no less than 8 wt.-% Mn, wherein an electric arc furnace with a basic lining is loaded with a charge and a slag-forming flux, which are thereafter melted.
An important slag-former is limestone, whereby a diffusion oxidation is performed before manganese is added to the melt. The manganese, as well as the slag mixture made of Al2O3, limestone, a carbon-containing material, iron silicide and calcium fluoride, are hereby added to the melt in small portions. The final deoxidation is performed with aluminum.
This process route is intended to reduce, on one hand, the phosphorus content in the melt and, on the other hand, the total content of manganese(II)-oxide and iron(II)-oxide in the refining slag. This process is particularly suitable to lower a high phosphorus content in the melt.
The conventional manufacturing approaches for producing steels with a high manganese content have fundamental disadvantages.
The high alloy contents require supply of additional heat to the melt during the addition of the charges that include the alloy elements; alloying, which is typically performed in a pan furnace, is very time-consuming due to the vessel geometry and the associated small heating power, and is therefore uneconomical.
In addition, alloying in the pan furnace is difficult, disadvantageous and uneconomical due to the large quantities of charges and slag-formers to be loaded and the associated small bath height or the low pan fill level at the beginning of the treatment.
Another problem is that large portions of the manganese contained in the charges can be absorbed by a rather acidic or oxygen-rich slag and are then no longer available to produce the melt.
The small manganese yield, meaning the quotient between the manganese contained in the melt and the manganese employed in the charges, additionally diminishes the cost-effectiveness.
Conversely, if a basic slag were selected, then foaming of the slag during addition of carbon to the melt would be insufficient.
The widespread practice of slag foaming in arc furnaces occurs as a result of the formation of CO/CO2 from the reaction of the added carbon with FeO from the slag. However, the quantity of FeO present in the basic slag is insufficient, because FeO is very efficiently reduced, for example by silicon from the melt. Formation of a foamed slag, however, increases the energy efficiency and prevents damage to the furnace lining.
In addition, the substances contained in the employed manganese carriers can cause the slag viscosity to increase during the alloying process and may even cause solidification of the slag.
The slag may also cause serious damage to the lining/refractory material, so that the slag path and the refractory material must be matched to one another.